On the calibration of a vectorial 4He pumped magnetometer

The prototype of a He-4 pumped vector magnetometer is presented. Large auxiliary coils systems used in previously developed apparatus to allow vector measurements from a scalar (atomic or nuclear resonance) sensor are replaced by a light triaxial modulation system associated with advanced techniques of signal processing. The performances of the helium scalar sensor are first briefly recalled; then the principle of the vector measurement, obtained by adding three (approximately) orthogonal modulations of different frequencies (all of the order of 10 Hz) is explained. Afterwards a second part of the paper is devoted to the calibration process, and a first estimate of the performances of the vector magnetometer is obtained. They confirm that this instrument could be a good candidate for an automatic absolute magnetic observatory: after the calibration process completion and a proper installation, it would provide by itself the absolute value of three orthogonal components of the field. In addition to that, the He-4 vector magnetometer appears to be also promising for space applications.